Sharing friends with your ex...

Is anyone else having any success at this?

 A few months ago a friend of mine started dating this girl and my ex became interested in her best friend. A few weeks later I met up with my friend and this girl and she and I really hit it off. We're both Bi and started seeing each other...in the meantime she stopped dating my friend (which had nothing to do with me) and started trying to work things out with her estranged husband, who was ok with her "dating" me, as was my fiance. Throughout all of this, my ex has been pursuing her best friend and has also maintained a close friendship with my girlfriend. I "broke up" with her today because of her close relationship with my ex...I can't seem to get her to see him for what he is. It's painful for me to hear her say that she thinks he's such a great guy and has been encouraging her best friend to go out with him. I don't want to bash him to her, especially when she seems to disregard everything I say about him, but I don't know how to tell her tactfully exactly WHY I can't be around her if she really feels this way about him. It really galls me that she doesn't seem to hear a thing I say. I can't tell her she has to choose, although that's pretty much what it boils down to. So I ended up cutting her loose and telling her I just need to put distance between him and me, which is true, and as such I think we need to lead separate lives with separate friends. Which is also true. She didn't understand and didn't take it very well. I don't know what else to say. I honestly feel that in continuing to be involved with her, I'm indirectly party to my ex becoming involved with her best friend, who has been through a LOT and doesn't need more pain. (See my My Story and blogs for the full story.) How can I get her to see that I can't be involved in that in any way, even if it's just hearing about it second hand?

She told me a few weeks ago that her loyalty is to me (and no, I didn't ask), but I don't see it. She met my ex first and I strongly suspect that she'd sooner give up my friendship than his.

I want to add that my relationship with her has turned into a pissing contest with my ex. I don't know if this is because he met her first and is being territorial, or if he's interested in her sexually too. He claims not to be, but has said several times that she is very hot and sexy. I think it's some of both.

Either way, this has become a conflict of interest. Has anyone else had a problem similar to this? At all??